1. Field of the Invention
This invention relates to a scanning optical apparatus and a multibeam scanning optical apparatus, and particularly to a scanning optical apparatus and a multibeam scanning optical apparatus suitable for use, for example, in an apparatus such as a laser beam printer (LBP) or a digital copying apparatus having the electrophotographic process adapted to deflect and reflect a beam of light optically modulated and emitted from light source means by a light deflector (deflecting element) comprising a rotatable polygon mirror or the like, thereafter optically scan a surface to be scanned through an imaging optical system having the fθ characteristic (fθ lens) and record image information.
2. Related Background Art
Heretofore, in the scanning optically apparatus of a laser beam printer or the like, a beam of light optically modulated and emerging from light source means in conformity with an image signal has been periodically deflected by a light deflector comprising, for example, a rotatable polygon mirror and has been converged into a spot-like shape on the surface of a photosensitive recording medium (photosensitive drum) having the fθ characteristic, and that surface has been optically scanned to thereby effect image recording.
FIG. 1 of the accompanying drawings is a schematic view of the essential portions of a scanning optical apparatus according to the prior art.
In FIG. 1, a divergent beam of light emitted from light source means 61 is made into a substantially parallel beam of light by a collimator lens 62, and the beam of light (the quantity of light) is limited by a stop 63 and enters a cylindrical lens 64 having predetermined refractive power only in a sub scanning direction. Of the parallel beam of light having entered the cylindrical lens 64, that part in a main scanning section intactly emerges in the state of a parallel beam of light. Also, that part in a sub scanning section converges and is imaged as a substantially linear image on the deflecting surface (reflecting surface) 65a of a light deflector 65 comprising a rotatable polygon mirror. Here, the main scanning section refers to a beam section the beam of light deflected and reflected by the deflecting surface of the light deflector forms with time. Also, the sub scanning section refers to a section containing the optical axis of an fθ lens and orthogonal to the main scanning section. The beam of light deflected and reflected by the deflecting surface 65a of the light deflector 65 is directed onto the surface of a photosensitive drum 68 as a surface to be scanned through an imaging optical system (fθ lens) 66 having the fθ characteristic, and the light deflector 65 is rotated in the direction of arrow A to thereby optically scan the surface of the photosensitive drum 68 and effect the recording of image information.
To effect the highly accurate recording of image information in a scanning optical apparatus of this kind, it is necessary that curvature of image field be well corrected over the entire area of a surface to be scanned and a spot diameter be uniform and that the angle and image height of the incident light have distortion (fθ characteristic) in which they are in a proportional relation. A scanning optical apparatus satisfying such optical characteristics or the correcting optical system (fθ lens) thereof has heretofore been variously proposed.
On the other hand, with the tendency of laser beam printers, digital copying apparatuses, etc. toward compactness and lower cost, similar things are required of the scanning optical apparatus.
As an apparatus which makes these requirements compatible, a scanning optical apparatus in which the fθ lens is comprised of a single lens is variously proposed, for example, in Japanese Patent Publication No. 61-48684, Japanese Laid-Open Patent Application No. 63-157122, Japanese Laid-Open Patent Application No. 4-104213, Japanese Laid-Open Patent Application No. 4-50908, etc.
Of these publications, in Japanese Patent Publication No. 61-48684 and Japanese Laid-Open Patent Application No. 63-157122, a concave single lens as on fθ lens is used on the light deflector side to converge a parallel beam of light from a collimator lens on the surface of a recording medium. Also, in Japanese Laid-Open Patent Application No. 4-104213, as fθ lenses, a concave single lens and a toroidal-surfaced single lens are used on the light deflector side and the image plane side, respectively, to make a beam of light converted into convergent light by a collimator lens enter the fθ lenses. Also, in Japanese Laid-Open Patent Application No. 4-50908, a single lens introducing a high-order aspherical surface into a lens surface is used as an fθ lens to make a beam of light converted into convergent light by a collimator lens enter the fθ lens.
However, in the scanning optical apparatuses according to the prior art described above, according to Japanese Patent Publication No. 61-48684, curvature of image field in the sub scanning direction remains and a parallel beam of light is imaged on the surface to be scanned, and this has led to the problem that the distance from the fθ lens to the surface to the scanned becomes a focal length f and is long and it is difficult to construct a compact scanning optical apparatus. In Japanese Laid-Open Patent Application No. 63-157122, the thickness of the fθ lens is great, and this has led to the problem that manufacture by molding is difficult and this makes a factor of increased cost. Japanese Laid-Open Patent Application No. 4-104213 has suffered from the problem that distortion remains. In Japanese Laid-Open Patent Application No. 4-50908, an fθ lens having a high-order aspherical surface is used and aberrations are corrected well, but there has been the problem that jitter of a period corresponding to the number of polygon surfaces occurs due to the mounting error of a polygon mirror which is a light deflector.
Further, problems common to these fθ lenses each comprised of a single lens has included the problem that due to the non-uniformity of the lateral magnification in the sub scanning direction between the light deflector and the surface to be scanned, the spot diameter in the sub scanning direction changes depending on image height.
FIGS. 2A and 2B of the accompanying drawings are cross-sectional views of the essential portions of a single beam scanning optical apparatus in the main scanning direction and the sub scanning direction, respectively, and show changes in the spot diameter (F number) in the sub scanning direction due to image height. In these figures, the same elements as the elements shown in FIG. 1 are given the same reference numerals.
Usually, in a plane inclination correcting optical system, it is necessary to bring the deflecting surface of a light deflector and a surface to be scanned into an optically conjugate relation (imaging relation) in order to optically correct the plane inclination of the deflecting surface. Accordingly, in an fθ lens having a predetermined lens shape in the main scanning section as in the aforedescribed examples of the prior art, lateral magnification is high on the axis (on-axis beam 21) as indicated at (1) in FIG. 2B, and lateral magnification becomes low off the axis (most off-axis beam 22) as indicated at (2) in FIG. 2B (there is also a case where this becomes converse depending on the lens shape in the main scanning section).
Thus, irregularity occurs to the lateral magnification in the sub scanning direction depending on the lens shape of the fθ lens in the main scanning plane thereof and a change in the spot diameter in the sub scanning direction due to image height occurs.
On the other hand, the ability of higher speed scanning is required of a scanning optical apparatus for use in an LBP because of the tendency of the LBP toward higher speed and higher accuracy, and from limitations such as the number of revolutions of a motor which is scanning means and the number of surfaces of a polygon mirror which is deflecting means, particularly the demand for a multibeam scanning optical apparatus capable of scanning a plurality of beams of light at a time is growing.
The above-described non-uniformity of the lateral magnification in the sub scanning direction makes the curve of the scanning line when the position of a light source (light source unit) is off the optical axis in Z direction indicated in FIGS. 2A and 2B and therefore, an optical system such as a multibeam scanning optical system (multibeam scanning optical apparatus) which scans a surface to be scanned at a time by the use of a plurality of beams of light off the optical axis has suffered from the problem that the scanning line bends on the surface to be scanned and as a result, the deterioration of image quality due to pitch irregularity occurs.